In the past, in ordinary engine control apparatus, a signal in synchronization with the rotation of an engine is used at the time of controlling the ignition timing, fuel injection, etc., of the engine, but in cases where ignition is not carried out in a normal manner due to the failure of an ignition control system, etc., a misfire may occur, so that unburnt gas can be discharged, thus leading to the occurrence of defect or trouble in an exhaust system.
In addition, upon the occurrence of a misfire, the reduction of the engine output and the reduction of engine stability will be caused, and besides, the unburnt gas will be discharged as it is, thus resulting in an increase of harmful components in the exhaust gas.
Further, in cases where the unburnt gas burns inside a three-way catalyst which is arranged in an exhaust passage, there is a problem that the temperature of the three-way catalyst goes up unusually, causing damage or deterioration of the three-way catalyst.
Accordingly, in order to avoid the above-mentioned problems, there has been proposed a misfire detection device for an internal combustion engine (for example, see a first patent document).
In the conventional misfire detection device described in the above-mentioned first patent document, the angular velocity of the engine is calculated from a deviation between the current rotation period and the last rotation period detected by an engine rotation period detecting means, and the angular velocity and the angular velocity deviation at the time of the occurrence of a misfire are adapted in advance for every engine operating state to adaptively prepare a misfire determination angular velocity determination value, so that in cases where the angular velocity of the engine during operation thereof is larger than the misfire determination angular velocity determination value thus adaptively prepared, a determination is made that a misfire has occurred.
On the other hand, in general purpose engines which are mounted on products such as an electric generator, a lawn mower, etc., it is known that it is desirable for the engine to be driven to operate at an air fuel ratio at which the output power of the engine becomes a maximum (a so-called output air fuel ratio) and which is richer than the stoichiometric air fuel ratio.
In addition in recent years, the strengthening of exhaust emission control values according to legal regulations is also promoted in general purpose engines from the point of view of environmental protection, and hence, it is required that the general purpose engines be each equipped with a three-way catalyst and an O2 sensor, similar to car or automobile engines, so that they are controlled at a stoichiometric air fuel ratio at which the purification rate of the three-way catalyst is high.
However, those equipment on which general purpose engines are mounted may be difficult to carry out periodical maintenance, and so, they are often continued to be operated, while malfunctions in the general purpose engines are not been noticed.
In such a case, if an operating person has continued to operate a general purpose engine, without noticing a malfunction thereof (e.g., a defect of an ignition system such as a spark plug, etc.), it will be concerned, similarly as stated above, that unburnt gases generated by misfiring burn in the three-way catalyst arranged in the exhaust passage to raise the temperature of the three-way catalyst in an abnormal manner, so that the three-way catalyst may be damaged, resulting in a remarkable reduction in the exhaust gas purification ability thereof.
In addition, the deteriorated state of the exhaust gas due to the damage to the three-way catalyst is not sensed or noticed by the operating person, and hence in the worst case, harmful components in excess of the exhaust emission control values may continue to be discharged into the atmospheric air.
Here, note that, as in the above-mentioned first patent document, a misfire detection device based on the fluctuation of the angular velocity of an engine has been proposed, but in the case of a general purpose engine, it is difficult to detect, because of the engine structure, the fluctuation of the angular velocity of the engine due to misfiring thereof.
This is because the general purpose engine, being used in a lower rotational speed range in comparison with ordinary engines, is designed to have a large rotational inertia so that a stable operation can be obtained at low speed rotation.
In view of such a structure, the fluctuation of the angular velocity of a general purpose engine at the time of misfire can not be easily detected, and products and equipment on which general purpose engines are mounted cover a wide variety of range, and hence, it is also difficult to specify loaded conditions in advance.